The present invention relates to an optical information-recording medium such as an optical disk, a reproducing apparatus and a recording and reproducing apparatus therefor.
An optical information recording system has lots of advantages such as to cope with various types of memories such as a read-only memory, a write-once memory and a rewritable memory, to record and reproduce information in a non-contact fashion, to easily remove an optical recording medium from a recording and reproducing apparatus and to carry an optical recording medium (optical recording medium is removable) and to execute random-access and to carry out recording and reproducing with a high recording density. Further, the optical information recording system is capable of realizing an inexpensive file of a large storage capacity, and therefore can be used in a wide field ranging from an industrial use to a commercial use.
Of the above types of memories, as optical memories of ROM (read-only memory), there have already been widely used various types of optical memories such as a digital audio disc (so-called compact disc (CD)), an optical video disk (so-called laser disk (LD)) and CD-ROM.
In these read-only optical memories, information recording pits are generally recorded on a transparent substrate as concave and convex shapes or portions in which optical constants are changed in the form of the pattern corresponding to this information signal Then, a reflecting layer made of a metal material such as Al (aluminum) is deposited on this recording pattern,
Reproducing light such as laser light is irradiated on such an optical recording medium from the side of the transparent substrate and the existence of the pits within the spot of reproducing light is identified by detecting the pits as an intensity of reflected light, whereby information is read out, reproduced from the optical recording medium
Then, in the above read-only optical memory, in order to maintain a recording capacity which is capable of coping with the advance of the technologies such as a digital VTR (digital video tape recorder) and a high-vision television (HDTV (high definition television)), there is an increasing demand for further improving a recording density of the read-only optical memory.
On the other hand, from a standpoint of improving operability, there is an increasing demand for miniaturizing an optical memory, ive., an optical recording medium From the above demand, it is also desired to further increase a recording density of an optical recording medium.
As a means for increasing the recording density of the optical recording medium, first, it is considered to microminiaturize the recording pattern, i e., to reduce the cycle of the above pit.
However, since the reproducing optical system has a diffraction limit xcex/(2N. A.) (xcex is the wavelength of reproducing light and N. A. is the numerical aperture of the objective lens of the optical system) in which the diameter of the beam spot cannot be reduced anymore, if the cycle of the pit is reduced too much, then there occurs the situation under which a plurality of pits are overlapping with each other within the spot of the reproducing light. There is then the disadvantage that an information signal cannot be reproduced from the optical recording medium.
Specifically, the reproducing apparatus has a cut-off spatial frequency of an MTF (modulation transfer function) which becomes an index of a resolving power which is determined based on the reproducing optical system For this reason, the numerical aperture N. A. of the objective lens of the optical system is increased in such a manner that the reproducing apparatus can cope with the recording pattern in which the cycle of the pits is short.
Further, it is attempted to improve a diffraction limit of reproducing light by using reproducing light having a short wavelength. At present, as this reproducing light, there is used laser light having a double higher harmonic wave (wavelength is 532 m) of a semiconductor laser excitation YAG laser or laser light having a double harmonic wave (wavelength is 425 nm) which results from directly converting laser light of a semiconductor laser by an SHG (secondary harmonic generation) element based on a nonlinear optical crystal. Further, there is used blue-violet semiconductor laser light having a wavelength of 400 nm which is generated by a nitride semiconductor InGaN-based semiconductor laser or the like.
Although there have been proposed or manufactured trial optical disks in which the recording layers of multilayers are laminated in order to further increase a recording capacity of the optical disk [K. Nishiuchi, Jpn. J. Appl Phys, Vol. 38, N03B, (1998), page 2163], if refractive indexes of respective layers become different, then a multiple beam interference of laser light occurs so that an interlayer crosstalk (intersymbol interference) tends to occur. Moreover, since it becomes difficult to focus light on the respective layers, transparent layers having a sufficiently large thickness of approximately several 10s of micrometers should be provided between respective recording layers Furthermore, if the number of layers increases, then reflected light is decreased from the distant layer so that it becomes difficult to obtain a satisfactory S/N (signal-to-noise ratio). Therefore, in actual practice, the number of multilayers is limited to several layers.
With a view to increasing a recording density and a storage capacity of an optical recording medium, there have been proposed an optical disk which makes effective use of fluorescence (edg., Japanese published patent application No. 7-77028) and an optical disk which makes effective use of thermoluminescence light (e.g., Japanese laid-open patent application No. 7-215871.
Further, there has been proposed an optical disk in which a cut-off spatial frequency can be increased about twice by detecting an optical disk in which micro-light-emission points are formed by filling a fluorescent substance into recording pits on the substrate by means of a confocal optical system so that recorded information can be reproduced with a higher resolving power (Japanese patent No. 2904434).
Furthermore, as a method of recording information in a three-dimensional fashion, there has been proposed a multilayer disk in which a plurality of light-emission layers are laminated through transparent mediums as information recording layers (U.S. Pat. No. 4090031, WO98/25262).
However, when a multilayer optical disk which makes effective use of light emission is realized, there arise the following problems
In the write-once optical disk which has heretofore been proposed, there are formed portions whose intensity of light emission is low by forming holes by heating fusion or sublimation or abrasion based on local heating of laser light or thermal deformation based on similar action or by locally changing the portions into other products whose luminous efficiency is lowered (or non-light-emission) based on thermolys is.
Furthermore, in a light-emission type optical disk in which a photochromic material used as a recording material and a fluorescent pigment are dispersed and mixed, there has been proposed a write-once type optical disk in which a recording sensitivity can be increased by using a mobility of energy between the photochromic material and the fluorescent pigment (Japanese patent No. 2,502,785).
In these cases, since the organic light-emission layer plays a role of a recording layer, there arise problems that there are few optical function materials which can meet with the above objects, the organic light-emission layer is inferior in durability and life-span, is unable to generate a sufficient luminous efficiency and whose detection signal is too small to maintain a sufficient S/N. Moreover, the above optical disk cannot be made rewritable.
Furthermore, there has been proposed a rewritable type multilayer optical disk in which a photon-mode recording can be executed based on a reversible photochemistry reaction by effectively utilizing a two-photon absorption process of a non-linear optical material (U.S. Pat. No. 5,268,862) In this case, in order to obtain ultrashort pulse light having a high peak power, a laser light source of a recording and reproducing apparatus becomes expensive, and an apparatus becomes large in size and complicated in structure due to an optical adjustment.
It is an object of the present invention to provide an optical recording medium which can realize an optical disk and an optical multilayer disk capable of generating a reproduced signal with a high S/N from the recording pattern of pits formed at a cycle shorter than a diffraction limit xcex/(2N. A.) of a reproducing optical system and a reproducing apparatus therefor and a recording and reproducing apparatus therefor.
According to an aspect of the present invention, there is provided an optical recording medium which is comprised of a laminated structure portion including a light-emission layer and a recording layer, wherein the light-emission layer is made of a light-emission material being excited by reproducing laser light to emit fluorescent light and the recording layer is made of a material in which a refractive index or an absorption coefficient relative to light emitted from the light-emission layer by recording laser light can be changed. The recording layer is disposed to be closer to a side into which there producing laser light is introduced than the light-emission layer.
Then, recorded information recorded by the change of the refractive index or the absorption coefficient of the recording layer is read out from the light-emission layer with irradiation of the reproducing laser light as the change of intensity of emitted light.
Then, in the optical recording medium according to the present invention, recording laser light and reproducing laser light relative to the recording layer and the light-emission layer are the same laser light and are selectively used by selecting their power.
Alternatively, the recording laser light and the reproducing laser light may be based on different laser light, i.e., laser light having different wavelengths
According to another aspect of the present invention, there is provided a reproducing apparatus which is a reproducing apparatus for the above optical recording medium according to the present invention. This reproducing apparatus is comprised of a reproducing laser light source for exciting the light-emission layer, a polarizing beam splitter, a quarter-wave plate, a dichroic mirror, an objective lens, a pinhole and a photodetector for detecting light emitted from the light-emission layer, wherein the light-emission layer of the optical recording medium is excited by irradiating the reproducing laser light on the optical recording medium through the polarizing beam splitters the quarter-wave plate, the dichroic mirror and the objective lens, Returned light, which is light emitted by the excitation and modulated by recording information based on the change of the refractive index or the absorption coefficient of the recording layer, is separated from the reproducing laser light by the dichroic mirror, detected by the photodetector through the pin-hole, and recorded information is read out from the recording layer
According to a further aspect of the present invention, there is provided a recording and reproducing apparatus which is a recording and reproducing apparatus for the above optical recording medium according to the present invention. This recording and reproducing apparatus is comprised of a laser light source for generating laser light which serves as recording laser light and the reproducing laser light relative to the recording layer, a polarizing beam splitter, a quarter-wave plate, a dichroic mirror, a pin-hole, an objective lens, and a photodetector for detecting light emitted from the light-emission layer, wherein laser light having a required recording power which is modulated in response to recording information is irradiated on the optical recording medium from the laser light source through the polarizing beam splitter, the quarter-wave plate, the dichroic mirror, and the objective lens to record information on the recording layer of the optical recording medium.
Further, the light-emission layer of this optical recording layer is excited by irradiating reproducing laser light having a power lower than the above recording power on the optical recording medium from the similar laser light source through the above polarizing beam splitter, the quarter-wave plate, the dichroic mirror and the objective lens. Recorded information can be read out from the recording layer by separating returned light of the excited light modulated by recording information obtained by the change of the refractive index or the absorption coefficient of the recording layer from the reproducing laser light with the dichroic mirror and detecting the same through the pin-hole by the photodetector.
As described above, in the optical recording medium and the reproducing apparatus or the recording and reproducing apparatus, the reproducing information signal light is not based on the returned light of the reproducing laser light but is based on light which results from modulating light emitted from the light-emission layer excited by reproducing laser light in response to recorded information of the recording layer.